Heat treatment of high-chromium alloys to improve ductility

ABSTRACT

Tensile ductility of high chromium-nickel (e.g., 50 percent chromium) and chromium-nickel-iron alloys improved when the alloys are subjected to an annealing treatment at a temperature of from about 600* C. to about 1,000* C. Incorporation of zirconium is quite beneficial in improving ductility, particularly at high chromium levels.

United States Patent Taylor et al.

[ Feb. 8, 1972 [S4] HEAT TREATMENT OF HIGH- CHROMIUM ALLOYS TO IMPROVE DUCTILITY [72] Inventors: Brian Taylor; Philip James Parry, Solihull,

, both of England [73] Assignee: The International Nickel Company, Inc.,

New York, NY.

221 Filed: Feb.7, 1969 211 Appl.No.: 797,679

[30] Foreign Application Priority Data Feb. 18, 1968 Great Britain ..6,290/68 U.S. Cl ..148/13, 148/1 1.5

2,157,060 5/1939 Schafmeister ..75/ 128 2,238,160 4/194l Doom ..75/171 2,809,139 l0/l957 Bloom et al... ..75/176 X 3,519,419 7/1970 Gibson et al. ..l48/32.5 X

FOREIGN PATENTS OR APPLICATIONS 607,975 l/l935 Germany ..75/l28 OTHER PUBLICATIONS Metal Progess,January 1940, pp. 64, 65, & 98

Primary ExaminerCharles N. Lovell Attorney-Maurice L. Pinel [57] ABSTRACT Tensile ductility of high chromium-nickel (e.g., 50 percent chromium) and chromium-nickel-iron alloys improved when the alloys are subjected to an annealing treatment at a temperature of from about 600 C. to about 1,000 C. Incorporation of zirconium is quite beneficial in improving ductility, particularly at high chromium levels.

6 Claims, No Drawings HEAT TREATMENT OF IIIGI-I-CHROMIUM ALLOYS TO IMPROVE DUCTILITY As is well known to those skilled in the art, it has long been recognized that high chromium alloys containing upwards of 30 percent chromium offer a most desirable combination of strength and resistance to various corrosive media. For example, alloys containing, say, 45 to 75 percent chromium, the balance being nickel, with or without iron, possess good strength and exhibit particularly outstanding resistance to the degradation effects of fuel ash at elevated temperatures. For such reasons alloys of this type can be used to advantage in furnaces, e.g., as thin-walled tubes, and also as welding wire for welding articles or components of furnaces and similar equipment.

, Equally well known, however, is the fact that such alloys are exceptionally difficult to work, though naturally their workability characteristics differ depending upon the percentage of chromium, being increasingly more difficult to work as the chromium content is increased. Paradoxically, the situation has been one in which the very constituent, chromium, largely responsible for the strength and corrosion resistant characteristics of such alloys is also the principal cause of the very tenuous commercial problem, poor workability. The severity of the problem is highlighted, for example, by reference to the binary chromium-nickel alloys in which even components of simple design are nearly always produced as castings since the alloys, being virtually unworkable, are extremely brittle. For example, it has not hitherto been generally possible to colddraw such alloys into wire or thin tubes, particularly in respect of alloys containing, say, upwards of 55 percent chromium, e.g., 60 percent to 70 percent chromium. Suffice to say, being limited to cast products is quite unattractive commercially in view of the fact that the ultimate product produced is restricted in size, shape, etc., as a result of the casting process.

It is true that, in general, workability is improved once the cast structure of an ingot has been initially broken down by some technique such as extrusion whereby the alloys are then placed in the wrought rather than cast state. However, even in this wrought condition the alloys still manifest low ductility. Attempts have been made in the past to increase room temperature ductility by heat treatment, and specifically wrought 50-50 chromium nickel alloys have been heated to a temperature of about l,200 C. or above and then water-quenched. This has not proved to be an acceptable panacea. Accordingly, the present invention in the main is directed to minimizing or overcoming the ductility problem but without an attendant sacrifice in other beneficial characteristics for which high chromium alloys have long been noted.

It has now been discovered that the ductility and workability of high chromium alloys in the wrought condition can be markedly improved upon subjecting the alloys to an annealing treatment over a special temperature range. It has also been found that in combination with the annealing treatment zirconium in alloys at the upper end of the chromium range exercises a most potent influence.

It is an object of the present invention to enhance the tensile ductility of chromium-nickel and chromium-nickel-iron alloys.

Generally speaking and in accordance with the present invention the room temperature ductility of high chromium alloys ascontemplated herein is significantly improved upon subjecting the alloys to an annealing treatment over the temperature range of from about 600 C., e.g., 625 C., to about 850 C., e.g., 800 C.; however, provided the alloys contain less than about 60 percent chromium a temperature of up to l,000 C. can be employed. The time required to produce cold-workability depends upon annealing temperature, generally being at least 16 hours or longer at 700 C. at least 100 hours, at a temperature of 600 C. and at least 8 hours at 850 C. Above the latter temperature shorter periods, say from 1 to 8 hours, can be used but the annealing period should be no more than one-half hour at 1,000 C. It is to be understood that the longer holding periods should be used in association with the lower end of the respective temperature ranges.

The alloys with which the invention is concerned contain from 45 to 75 percent chromium, up to 46 percent iron and the balance nickel. As will be understood by those skilled in the art, the term balance" or balance essentially used herein in referring to the nickel content does not exclude the presence of other elements such as those commonly present in incidental elements, e.g., deoxidizing and cleansing elements. and impurities normally associated therewith, in small amounts which do not adversely affect the basic characteristics of the alloy.

In connection with the foregoing, the alloys may contain carbon up to 0.2 percent, and as usual small amounts of manganese, silicon and iron not exceeding 2 percent as incidental elements. The usual impurities, which include sulfur and phosphorus up to 0.02 percent each, may, of course, be present. Nitrogen is an impurity, and it is found that this element may have an adverse effect on hot workability, so the amount of it should be kept as low as possible and is preferably below 0.2 percent. In practice we find it difficult to reduce the nitrogen content below 0.005 or 0.0l percent. To ensure that the nitrogen is kept to a suitably low value the alloys are preferably made by vacuum melting, but they may be made by air melting and in this case we prefer to cover the surface of the molten metal by a suitable slag and shield it by an inert gas. The nitrogen usually enters the melt from the chromium and it is therefore important to use chromium with a low nitrogen content. Preferably, we use chromium made by the aluminothermic process and having a low nitrogen content, e.g., about 0.01 percent.

In addition, the alloys advantageously contain an element that forms a eutectic with one or both of the chromium and nickel. Such alloys are described in our US. application Ser.

7 No. 797,515, filed on Feb. 7, 1969. As mentioned therein, the

eutectic-forming element is advantageously zirconium which is beneficially present in an amount such that a eutectic of nickel-zirconium with chromium in solution is formed in the cast alloy. Generally speaking, the content of zirconium should be at least 0.05 percent and it may be up to 4 or even 6 percent. In lieu of zirconium, yttrium, hafnium or cerium can be used as the eutectic-forming constituent.

In utilizing a eutectic-forming element referred to above, care must be exercised regarding the actual percentage thereof since they display a very considerable affinity for such constituents as nitrogen. Nitrogen, though not an element normally mentioned in specifications of chromium-nickel and chromium-nickel-iron alloys is in fact invariably present as an impurity. Typically, alloys of the type in question contain up to 0.2 percent nitrogen, although at times it runs as high as 0.4 percent. To form the desired eutectic there must be an amount of zirconium or other eutectic-froming element in excess of the amount which will combine with the nitrogen to form the corresponding nitride (and this would be applicable with regard to other elements with which the eutectic forming element would combine if present in the alloy). This excess amount is referred to herein as the effective zirconium (or cerium, yttrium or hafnium). Thus, for zirconium the amount combined as nitride is calculated as 6.5 times the nitrogen content, meaning, if 0.] percent nitrogen is present there must be at least 0.65 percent zirconium to counteract the nitrogen effect. Similar amounts of effective yttrium, hafnium and cerium are required, but the amount of each of these elements combined as nitride differs. The effective cerium is that in excess of 9 times the nitrogen content, the effective yttrium that i in excess of 6 times the nitrogen content and the effective hafnium that in excess of 13 times the nitrogen content.

When the zirconium or other eutectic-forming element is added to a molten chromium-nickel alloy some of it is lost. Assuming that the element is zirconium, the alloys may be made by melting the chromium and nickel; ascertaining the nitrogen content of the melt by tapping the melt and determining the residual nitrogen by rapid vacuum or inert-gas fusion techniques; and adding zirconium in an amount based on calculation after the determination of the nitrogen content. When the melting is effected in air, we may add up to three times the calculated zirconium content to the melt in order to produce a desired effective zirconium content. In vacuum melting and casting we add less. Since the presence of the desired eutectic of nickel and zirconium is readily ascertainable under microscopic examination in the as-cast alloy, its existence may readily be ascertained by taking a sample of the melt before casting and preparing a section, and examining it under the microscope.

in carrying the invention into practice, it should be mentioned that when a product form such aswire is being colddrawn, intermediate annealing between passes is required. The duration of each intermediate anneal may be much less than that of the first anneal required to impart good drawability. For example, the starting material in the form of hot-extruded rod may initially be annealed for 16 hours at 750 C. or 800 C., but after the first cold reduction by wire-drawing, which may reduce the section of the rod up to 40 percent With regard to the results reported in Table l, the alloys which nominally contained 50 percent chromium although they could be cold-drawn without the annealing stepof the invention (since for cold-drawing an elongation of at least 10 percent and a reduction of area of at least percent may be regarded as satisfactory), nonetheless, the annealing treatment of the invention considerably increased the reduction of area. Moreover, it will be observed that the alloys which nominally contained 60 and 70 percent chromium could not be drawn at all in the extruded state, but were rendered colddrawable by the invention, although those nominally containing 70 percent chromium required the presence of zirconium also.

In Table ll, the details are set forth concerning the colddrawing of rod produced from an alloy which nominally contained 60 percent chromium and extruded as described above in connection with the alloys of Table l. The rod was annealed TABLE II Alloy composition, wt. percent Anneal, Hard- Initial Final Hardhrs. at temp., ness, diameter, diameter, R. of A., ness, 0 N Z1 Cr Ni C. Hv inch inch percent Hv30 0. 021 0. 063 l. 15 60 Bal As extruded 480 24 800 317 0. 340 0.280 32. 3 420 24/800 286 0. 280 0.220 42. 2 3 0. 220 0. 112 74 440 0.112 0. 056 75 0.056 0. 045 35. 7 0.056 0.050 20.4 0. 048 1 26.

0.021 0. 063 Nil 0. 150 Could not be drawn, failed at point 0.021 0. 063 1. 15 60 Bal %/1, 100

1 Broken. 2 Center.

3 Outer edge. reduction in area or more, the annealing time needed to eliminate work-hardening and to recover the good workability for further reduction need be no more than about 2 hours.

For the purpose of giving those skilled in the art a better appreciation of the invention, the following illustrative data are given:

A series of alloys nominally containing 50 60 and 70 percent chromium, respectively, were prepared, the balance of the composition being essentially nickel with or without zir' conium in various amounts. All the alloys were made by vacuum melting and casting, thereafter being hot-worked at 1,1 20 C. by extrusion using an extrusion ratio of 8:1

In Table l there is given the chemical composition of each alloy, the tensile properties of the extruded rods, and the properties after the rods were annealed at 700 C. for a period of time between 60 and 70 hours.

and then drawn with five intermediate annealing treatments, different specimens being subjected to different treatments after being drawn to 0.056 inch diameter.

The hardness, l-lV-30, of the rod as extruded was 480, after annealing for 24 hours at 800 C. was 317. it will be seen that this value increased to 420 in the course of the first reduction. With regard to the last intermediate anneal which consisted of 2 hour holding at 750 C., the rod was thereafter satisfactorily reduced in the last reduction to 0.045 inch in diameter; however, when instead one of the specimens was subjected to annealing for one hour at 1,120 C. before the last reduction, it broke in the final pass. Annealing at 1,100" C. for '24 hour resulted in failure.

Additional data is reported in Table 111 concerning a rod formed from an alloy which nominally contained 70 percent chromium cold drawn in the same general manner as the zir- TABLE. 1

Room temperature tensile properties Extruded for 60-70 hrs. As extruded at 700 0. Chemical composition, weight percent U.T.S., Elong., R. of A., U.T.S., E1ong., R. 0111., Fe O N Zr Cr Ni t.s i percent percent t.s.i. percent percent 0. 011 0. 022 Nil 49. 6 Bal. 62 23 46 68 25. 5 64 0. 01 0. 022 0. 08 49. 5 139.1. 67. 5 21. 6 32 68. 8 24 64 0.011 0. 022 1. 15 49. 5 Hal. 62. 4 21. 6 30 66. 4 24 49 9. 7 0. 040 0. 049 1. 05 49. 5 Bal. 88. 0 9. 3 84. 9 15 0. 021 0. 070 Nil 49. 5 Bal. 72 24. 8 37 69. 6 23 42 0. 021 0. 070 0. 24 49. 5 Hal. 72 21 35 70. 4 23, 5 53 0. 021 0. 070 1. 05 49. 5 Bal. 62. 4 21 32 64 23 9. 4 0. 040 0. 058 1. 49. 2 Bal. 108. 8 1. 0 52. 4 1. 5 0. 021 0. 068 N11 60. 0 132.1. 106. 4 0 0 72. 0 19 35 0. 021 0. 063 0. 27 60. 0 Ba]. 102. 4 0 0 73. 6 23 47 0. 021 0. 063 1. 15 60. 0 B211. 98. 4 0 0 69. 6 22 44 0. 020 0. 040 1 1. 83 46. 5 Bal. 58. 5 5.0 72.0 16 0. 006 0. 060 Nil 60. 3 Bal. 104 0 0 72.0 21 37 0. 006 0.060 0. 30 60. 3 Bal. 103. 2 0 0 74. 4 22 35 0. 006 0. 060 1. 60. 3 Bal. 100. 8 0 0 72 20. 4 38 0. 037 0.040 2 3. 73 41. 6 Hal. 74. 2 4. 4 82. 4 12. 5 0. 005 0. Nil 60.0 132.1. 104 0 0 70. 8 16 45 0. 005 0. 110 0. 25 60. 0 Bal. 107. 2 0 0 72. 8 18 32 0. 005 0. 110 1. 70 60. 0 Bal. 96. 8 0 0 68. 8 19 40 N.D. 0.053 Nil 69.8 Bal. 82.4 0 N.D. 81.6 2.1 N.D. N.D. 0.053 0.08 69. 8 Hal. 84 0 N.D. 79. 8 13.2 N.D. N.D. 0. 053 1. 25 69. 6 Bal. 80. 2 0 N.D. 78. 4 11. 9 N.D. N.D. 0. 14 Nil 71. 4 Bal. 66. 4 0 N.D. 73. 6 1. 7 N.D. N.D. 0.14 0.78 71. 0 Bal. 106. 4 0 N.D. 76 13. 8 N.D. N.D. 0. 14 1. 40 70. 0 Bal. 53. 6 0 N.D. 78. 4 10. 6 N.D.

N0'iE.Bal.=Ba1ance essentially nickel and impurities; U.T.S.=Ultimate tensile strength; t.s.i.=Long tons per sguare inch; Elong.=Elong-ation; R. of A. =Reduction of area; N.D.=Not determined.

1 Yttrium. 1.. Hainiu The illustrative data given in Table III confirms that long holding periods at the highest temperatures, e.g., 24 hours at 1,000 C., is detrimental in contrast to the same holding period at a somewhat lower temperature.

Consideration of all the foregoing data reflects that the room-temperature ductility of high chromium alloys up to approximately 75 percent chromium is considerably improved as a result of the heat treatment contemplated herein. As to other constituents, the presence of zirconium has been shown to be of particular advantage at chromium levels of 70 percent. At least 0.05 and up to L percent zirconium, e.g., 0.5 to 1.5 percent, should be present when the chromium is 65 percent or higher. Known alloys containing about 50 percent chromium and 50 percent nickel are particularly amenable to the benefits conferred by the subject invention.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A process for improving the workability and tensile ductility of high chromium, nickel containing alloys, said alloys containing at least about 30 percent and up to about 75 percent chromium, up to about 46 percent iron, the balance being essentially nickel and at least one element selected from the group consisting of zirconium, yttrium, hafnium and cerium in an amount sufficient to fonn said eutectic phase, which comprises subjecting such alloy to an annealing treatment over the temperature range of about 600 to 1,000 C. with, however, the following provisos: (a) the annealing temperature does not exceed about 850 C. when the chromium content is about 60 to 75 percent, (b) the annealing period does not exceed about 1% hour at a temperature of about l,000 C., is at least about hours at 600 C., is at least about 16 hours at 700 C. and is at least 8 hours at about 850 C.

2. A process in accordance with claim 1 in which an effective amount of zirconium is present sufficient to form a nickel eutectic when the chromium content is at least 65 percent.

3. A process in accordance with claim 1 in which the alloy contains from 45 to 75 percent chromium.

4. A process in accordance with claim 1 in which when the temperature is much above 850 C. the holding period does not exceed about 8 hours.

5. A process in accordance with claim 1 in which an effective amount of zirconium is present. 

2. A process in accordance with claim 1 in which an effective amount of zirconium is present sufficient to form a nickel eutectic when the chromium content is at least 65 percent.
 3. A process in accordance with claim 1 in which the alloy contains from 45 to 75 percent chromium.
 4. A process in accordance with claim 1 in which when the temperature is much above 850* C. the holding period does not exceed about 8 hours.
 5. A process in accordance with claim 1 in which an effective amount of zirconium is present.
 6. A process in accordance with claim 4 in which the effective zirconium is from 0.5 to 1.5 percent. 